Future low emissions standards for vehicles are forcing automobile and catalyst manufacturers to focus on reducing cold start hydrocarbon emissions since a large portion of hydrocarbon emissions occur during the cold start period. Consequently, control of emissions during the cold start operation of a vehicle containing an internal combustion engine is essential. Vehicles equipped with a conventional three-way catalytic converter typically contain precious metals supported on a washcoat layer, which in turn is deposited on a monolithic carrier. Fresh catalysts start to operate at about 170° C., while aged catalysts work only at about 200-225° C. These catalysts usually require at least 1-2 min to reach such temperatures, and during this “cold start” period, 70-80% of the tailpipe hydrocarbon emissions occur. Such cold start emissions often result in failure in the cycle of the U.S. federal test procedure (FTP), a standardized laboratory method for new vehicles testing that is based on two simulated environments; namely, city and highway, in which prototypes of new vehicle models are driven by a trained driver in a laboratory on a dynamometer. At lower temperatures where the catalyst in a catalytic converter is not able to effectively convert incompletely burned hydrocarbons to final combustion products, a hydrocarbon adsorber system should trap hydrocarbons exhausted from the engine before they reach the catalytic converter by adsorbing the incompletely burned hydrocarbons. In the ideal case, desorption should occur at temperatures exceeding catalyst light-off.
The critical factors for any emission hydrocarbon trap are the adsorption capacity of the adsorbent, the desorption temperature at which adsorbed hydrocarbons are desorbed and passed to the catalytic converter (must be higher than the catalyst operating temperature), and the hydrothermal stability of the adsorbent. Molecular sieves such as zeolites have generally been found to be useful adsorbents for this application in part due to their hydrothermal stability under these conditions compared to other materials.
Various studies have focused on the use of molecular sieves, and zeolites in particular, as adsorbents, including medium and large pore zeolites, although, in some cases, the types of molecular sieves or zeolites used have not been identified. A series of zeolites (β, ZSM-5, mordenite, and Y) have been investigated in such studies for their hydrocarbon adsorption capacity under a variety of conditions (see, e.g., Burke, N. R.; Trimm, D. L.; Howe, R. F. Appl. Catal., B 2003, 46, 97; Lafyatis, D. S.; Ansell, G. P.; Bennett, S. C.; Frost, J. C.; Millington, P. J.; Rajaram, R. R.; Walker, A. P.; Ballinger, T. H. Appl. Catal., B 1998, 18, 123; Noda, N.; Takahashi, A.; Shibagaki, Y.; Mizuno, H. SAE Tech. Pap. Ser. 1998, 980423; and, Czaplewski, K. F.; Reitz, T. L.; Kim, Y. J.; Snurr, R. Q. Microporous Mesoporous Mater. 2002, 56, 55)
Previous investigations have thus far found zeolite-β to be a promising material for this application. However, aged zeolite-β catalysts demonstrate degraded performance in trapping hydroccarbons due to low hydrothermal stability when used as an exhaust gas adsorbent. Hence, despite advances in the art, an important need continues to exist for a material that possesses a better adsorption capacity, higher desorption temperature, and hydrothermal stability than current adsorbents such as zeolite-β for use in emission control, particularly during the cold start operation of an internal combustion engine.